Alternately, simply mix one cup of brown sugar (dark's the best in my opinion) with 1/2 cup of any good mustard and coat the roasting ham (forgive me, but being an American I'm not sure what "gammon" is).
Leave the ham uncovered for the last hour of roasting/baking. Makes a nice crust as well as adding the flavor.
Mrs. C likes to add about 1/2 cup of ketchup with the brown sugar/mustard mix, but every one's to kind to tell her she's a pagan... but, each to his/her own...